Did solar winds cause watery Mars to dry up?

"The removal of atmosphere from Mars by episodic extreme events may have been very important over Mars' history, just as a single tsunami can remove a portion of the ocean shore that would have taken millennia to erode by the steady lapping of the tides," says Jasper Halekas. (Credit: NASA)

Billions of years ago, Mars was a warm and watery planet. Now scientists believe they are closer to understanding why things changed.

Using data from NASA’s MAVEN (Mars Atmosphere and Volatile Evolution) mission, researchers determined the rate at which stripping by the solar wind is causing the Martian atmosphere to currently lose gas to space. Scientists say this loss may have been an important part of why the Martian climate went from one that might have been able to support life at the surface to the cold, dry, desert planet we see today.

The sun plays a role in the planet’s loss of atmosphere and water to space, by the solar wind accelerating ions (electrically charged gas atoms) in the Martian upper atmosphere and stripping them from the planet altogether. Powerful solar storms, called coronal mass ejections, can multiply the ions’ escape by as much as a factor of 20.

Further, coronal mass ejections are able to produce such a pronounced effect because solar storms collapsed the Martian magnetosphere, reducing its size by approximately 600 miles, and exposing more of the planet’s atmosphere to the solar wind.

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“The removal of atmosphere from Mars by episodic extreme events may have been very important over Mars’ history, just as a single tsunami can remove a portion of the ocean shore that would have taken millennia to erode by the steady lapping of the tides,” says Jasper Halekas, associate professor in physics and astronomy at the University of Iowa.

The first scientific results from the MAVEN mission were published online in a series of papers in the journals Science and Geophysical Research Letters.

MAVEN measurements have demonstrated that the solar wind is able to strip away gas at a rate of about 100 grams (equivalent to roughly 1/4 pound) every second. The loss comes from three different regions around the planet: above the Martian poles in a “polar plume”; down the “tail,” where the solar wind flows behind Mars; and from the extended cloud of gas that always surrounds Mars.

A series of dramatic solar storms hit Mars’ atmosphere in March 2015, and MAVEN found that the loss was accelerated. The combination of greater loss rates and increased solar storms in the past suggests that loss of atmosphere to space was likely a major process in changing the Martian climate.

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“Like the theft of a few coins from a cash register every day, the loss becomes significant over time,” says MAVEN Principal Investigator Bruce Jakosky of the University of Colorado. “More importantly, with MAVEN we’ve seen that the atmospheric erosion increases significantly during solar storms.

“So we think that the loss rate was much higher billions of years ago when the sun was young and more active. As a result, we expect that loss of atmosphere to space was likely to have been a major process in changing the Martian climate.”

The mission’s goal is to determine how much of the planet’s atmosphere and water has been lost to space. Scientists are interested in the history of liquid water on Mars because it has had a major geological impact on the surface over time and because it is necessary for life as we know it to exist at the surface.

MAVEN has been operating at Mars for just over a year and will complete its primary science mission on Nov. 16.

Source: University of Iowa